Many types of electronics systems contain memory. A common, low-cost memory is dynamic-random-access memory (DRAM). Rather than sell individual DRAM chips, such chips are often pre-assembled into modules that can be inserted into sockets in a wide variety of systems. Such memory modules have gained enormous popularity in recent years. Most personal computers (PC's) are shipped with sockets for memory modules so the PC user can later add additional modules, increasing the memory capacity of the PC. High-volume production and competition have driven module costs down dramatically, benefiting the PC buyer.
Memory modules are made in many different sizes and capacities, such as older 30-pin and 72-pin single-inline memory modules (SIMMs) and newer 168-pin, 184-pin, and 240-pin dual inline memory modules (DIMMs). The “pins” were originally pins extending from the module's edge, but now most modules are leadless, having metal contact pads or leads. The modules are small in size, being about 3-5 inches long and about an inch to an inch and a half in height.
The modules contain a small printed-circuit board substrate, typically a multi-layer board with alternating laminated layers of fiberglass insulation and foil or metal interconnect layers. Surface mounted components such as DRAM chips and capacitors are soldered onto one or both surfaces of the substrate.
FIGS. 1A-B show a double-sided memory module. Memory module 20 contains a substrate such as a multi-layer printed-circuit board (PCB) with surface-mounted DRAM chips 22 mounted to the front surface or side of the substrate, as shown in FIG. 1A, while more DRAM chips 26 are mounted to the back side or surface of the substrate as shown in FIG. 1B. Metal contact pads 12 are positioned along the bottom edge of the module on both front and back surfaces. Metal contact pads 12 mate with pads on a module socket to electrically connect the module to a PC's motherboard. Holes 16 and notches are present on some kinds of modules to ensure that the module is correctly positioned in the socket. Capacitors 14 or other discrete components are surface-mounted on the substrate to filter noise from DRAM chips 22, 26.
Flash memory 24 is an electrically-erasable programmable read-only memory (EEPROM) that can be programmed by the manufacturer. Configuration information about the memory is often written to flash memory 24. Such configuration information may include the memory depth and width, locations of bad memory cells, speed of the memory, the type of memory, a manufacturing week code, critical timing parameters such as clock rate, CAS (column address select) latency, and any proprietary information. Flash memory 24 is known in one embodiment as a serial-presence detect (SPD) memory. During boot-up, the PC checks for “serial presence” on its memory bus. If a serial presence is detected, the PC reads the stored memory configuration through the serial line. Once programmed, flash memory 24 can be read by a PC on boot-up to determine the kind of memory installed in the PC.
A manufacturer may produce several different kinds of memory modules for sale. For example, one kind of memory module may contain ×4 DRAM chips that are 4 data bits wide, while a second kind of memory module may contain ×8 DRAM chips that are 8 bits wide. Some customers may order memory modules with the ×8 DRAM chips, while other customers order memory modules with ×4 DRAM chips. Predicting the exact product mix of ×4 and ×8 chip modules can be difficult, and incorrectly predicting the product mix may lead to increased inventory cost or delayed or lost sales.
What is desired is a memory module substrate board that can support either ×4 or ×8 DRAM chips. A memory module board that can have DRAM chips of varying data widths mounted thereon is desirable. A single memory module PCB that can be used to manufacture several kinds of memory modules is desirable.